Field of the Invention
The present invention relates to an electrochemical quantitative analytic tool for in situ monitoring of monochloramine in aquatic systems and for measurement of monochloramine concentration profiles within biofilm for microscopic study of its kinetics.
The Prior Art
Monochloramine (NH2Cl) has been used in low concentrations as a secondary disinfectant in treatment of municipal water supplies as an alternative to chlorination. Monochloramine is more stable and does not dissipate as rapidly as free chlorine. However, while the tendency of monochloramine to convert organic materials in the water to carcinogens, e.g., chloroform and carbon tetrachloride, is much lower than that of free chlorine, it remains a concern.
Many water utilities have switched from free chlorine to monochloramine as a secondary disinfectant to comply with disinfectant by-product (DBP) regulations because of the tendency of monochloramine to form lower levels of DBPs. A nitrification risk is associated with chloramine addition due to ammonia addition for chloramine formation and ammonia's subsequent release during chloramine decay. Nitrification in drinking water distribution systems may result in water quality degradation and non-compliance with existing regulations. It has been established that nitrifying biofilm is involved in nitrification episodes in water utilities. However, the present understanding of distribution system nitrification and its control is incomplete. In addition, microscopic biofilm research on disinfectant penetration and nitrification has been limited due to the lack of necessary tools. Microelectrode sensor techniques, e.g., the present invention, are required to profile chemical constituent transport and reaction and to monitor growth and inactivation, elucidating nitrification within distribution system biofilm. The ability to monitor at the microscopic level is in great demand in various research and development areas in biochemistry and environmental engineering. To evaluate nitrification and nitrifying biofilm control strategies in water utilities, information on disinfectant biofilm penetration and its effect on biofilm activity, viability, and recovery between monochloramine and free chlorine is required. A requirement met by the present invention.
Several researchers have reported on electrochemical monochloramine sensors using a platinum or gold disk (or electrode) or platinum wire. Previous work using platinum microelectrodes showed dissolved oxygen (DO) interference during monochloramine measurement, resulting in generation of multiple calibration curves. For example, FIG. 5 shows a 3-D calibration curve for a modification of the chloramine sensor disclosed by W. H. Lee, D. G. Wahman, P. Bishop, and J. G. Pressman (2011) “Free chlorine and monochloramine application to nitrifying biofilm: comparison of biofilm penetration, activity, and viability.” Environmental Science and Technology, 45, 1421-1419. FIG. 6 shows multiple calibration curves obtained using a modified version of the device of W. H. Lee, J. G. Pressman, D. G. Wahman, and P. L. Bishop (2010) “Characterization and application of a chlorine microelectrode for measuring monochloramine within a biofilm.” Sensors and Actuators B,” 145(2), 734-742, indicative of DO interference.
Chloramine amperometric sensors with a noble metal in non-compact form are disclosed in U.S. Pat. No. 7,087,150 and in Canadian Patent No. 2482011. The non-compact form may be a gas diffusion electrode, which can include metal mesh, carbon paper, carbon cloth, metal/carbon powder loaded on a porous membrane, or any combination thereof. However, such electrodes are too large, i.e., a 0.41 mm diameter platinum wire or gold mesh and a 25.1 mm overall diameter, to apply at the small scale (˜10 microns) required for microscopic study, e.g., in situ monitoring of monochloramine biofilm penetration both spatially and temporally. Applicants know of no commercially available miniaturized needle type chloramine sensitive sensor. Further, conventional sensors limit the access of reduced chloramine species to the sensing electrode (cathode) due to the geometry of the sensing electrode; therefore, sensor linearity decreases at high chloramines concentrations, e.g. 2 mg L−1. Another problem with currently available chloramine amperometric sensors is, as mentioned above, dissolved oxygen (DO) interference during monochloramine measurement. The present inventors have experienced dissolved oxygen interference during monochloramine measurement using a prior art platinum microelectrode, which required generation of a 3D surface calibration curve (response vs. DO vs. monochloramine concentration) or two calibration curves with different DO concentrations., e.g., 0% DO and 21% fully saturated DO, significantly limiting potential use of the microelectrode.
Relevant publications for understanding the present invention (e.g., sensor application to biofilm study and data interpretation), in addition to the foregoing, include:                D. A. Davies, “Anodic Voltammetric Determination of Monochloramine in Water” M. S. thesis, University of Wisconsin, Milwaukee, Wis., USA, 1985.        A. N. Tsaousis, “Amperometric Determination of Hypochlorous Acid and Monochloramine at Gold Electrodes,” M.S. thesis, University of Wisconsin, Milwaukee, Wis., USA, 1985        B. Piela, P. K. Wrona, “Electrochemical Behavior of Chloramines on the Rotating Platinum and Gold Electrodes.” J. Electrochem. Soc. 150 (5) (2003) 255-265.        